Thermal Rearrangement of Azulenes to Naphthalenes: A Deeper Insight into the Mechanisms

Abstract

The thermal rearrangement of azulene to naphthalene has been the subject of several experimental and computational studies. Here, we reexamine the proposed mechanisms at the DFT level. The use of different functionals showed that the HF-exchange contribution significantly affects reaction energies and barrier heights. Accordingly, all proposed pathways were investigated with the optimal method, M06-2X/6-311+G­(d,p), which confirms the norcaradiene–vinylidene mechanism (A) as the dominant unimolecular route (E a ≈ 76 kcal/mol) able to account for the major products of pyrolyses using 13C- or substituent-labeled azulenes. Moreover, a facile vinylidene–acetylene interconversion will scramble the terminal carbon atoms in the vinylidene. Several other potential intramolecular reaction mechanisms (B–E) are ruled out because of higher activation energies (>84 kcal/mol) and failure to reproduce the results obtained with substituted and 13C-labeled azulenes and benzazulenes. These experimental results also demonstrate that the proposed free radical or H atom-induced intermolecular methylene walk and spiran mechanisms cannot be major contributors, especially under flash vacuum pyrolysis conditions.

Azulene–Naphthalene, Naphthalene–Naphthalene, and Azulene–Azulene Rearrangements

“…The earliest example, the thermal rearrangement of azulene to naphthalene (A–N), was discovered by Heilbronner and colleagues in the 1940s. − Several subsequent investigations using a variety of thermolysis techniques , aimed at a clarification of the reaction mechanism. , In addition, theoretical investigations have been performed to shed light on the mechanisms. Thus, we recently explored mechanistic scenarios for the azulene → naphthalene rearrangement (A–N) with the aid of density functional theory (DFT) calculations and examined their ability to explain the outcome of experiments with 13 C- and substituent-labeled azulenes . The unimolecular norcaradiene–vinylidene mechanism (NVM) with an activation energy of ∼76.5 kcal/mol (74.6 kcal/mol at the CCSD­(T) level) can account for all major products (Scheme ).…”

“…It is notable that a similar reaction path to form isoaromatic naphthalene by direct electrocyclzation has been considered previously and shown to have an activation energy of ∼69 kcal/mol. 8 3.2. Dehydrotriquinacene (DTQ) Mechanism.…”

“…The mechanism is supported by an activation energy of 65−76 kcal/mol and the ability to account for the major products of pyrolysis of 13 C-labeled and substituted azulenes. 8 As indicated in Scheme 7A, the pathway includes reversible valence isomerization of the vinylidene 28 to acetylene 29 through either a 1,2-H shift or terminal carbon exchange, 29→30. The latter step eventually results in the exchange of C1 and C8a in the formed azulene, in agreement with the results of flow pyrolysis of 1-I3 C-azulene at 700−900 °C for 0.3−3.0 min.…”

“…Here, the most promising is the spiran mechanism catalyzed by free radicals or H atoms (Figure S4), but in some cases it predicts products that have not been found experimentally. 8 Furthermore, no direct experimental evidence for this mechanism has ever been reported. Scheme 13 indicates the pathway by which the spiran mechanism could explain the rearrangement 13 → 69 → 14 described in Figures 4 and 5 above.…”

“…Thus, we recently explored mechanistic scenarios for the azulene → naphthalene rearrangement (A−N) with the aid of density functional theory (DFT) calculations and examined their ability to explain the outcome of experiments with 13 C-and substituent-labeled azulenes. 8 The unimolecular norcaradiene−vinylidene mechanism (NVM) 9 with an activation energy of ∼76.5 kcal/mol (74.6 kcal/mol at the CCSD(T) level) can account for all major products (Scheme 1). Furthermore, we indicated that carbon scrambling by vinylidene−acetylene interconversion requires <70 kcal/mol, thus making it very likely to occur during the norcaradiene− vinylidene rearrangement.…”